A 3D printed object has to be able to support the weight of its own layers, as objects that are too fluid can collapse under their own weight. That’s why it can be so difficult to 3D print with viscous materials. A fluid’s viscosity is a measure of its resistance to gradual deformation by shear or tensile stress. This corresponds to the concept of thickness in terms of liquid; therefore, thick, semi-fluid honey is more viscous than fluid water.

But thanks to some new research coming out of Purdue University, it’s possible to achieve fine precision 3D prints using extremely viscous materials with the consistency of cookie dough and clay.

“It’s very exciting that we can print materials with consistensies that no one’s been able to print. We can 3D print different textures of food; biomedical implants, like dental crowns made of ceramics, can be customized. Pharmacies can 3D print personalized drugs, so a person only has to take one pill, instead of 10,” said Emre Gundez, an Assistant Research Professor in the university’s School of Mechanical Engineering.

While other solutions to the inherent issues of 3D printing with viscous materials actually call for a change in the material’s composition, the Purdue researchers tried something completely different – applying high-amplitude ultrasonic vibrations directly to the 3D printer’s nozzle. Ultrasonic manipulation can lead to unique innovations for 3D printing.

Purdue University assistant professor Emre Gunduz used ultrasonic vibrations to maintain a flow of the material through the printer nozzle. [Image: Jared Pike, Purdue University]

Gundez explained, “We found that by vibrating the nozzle in a very specific way, we can reduce the friction on the nozzle walls, and the material just snakes through.”

This breakthrough solution could one day lead to the 3D printing of biomedical implants, customized ceramics, food (who wouldn’t want their own cookie dough 3D printer?), pharmaceuticals, and even solid rocket fuel. The team is conducting its research at Zucrow Labs on campus, which is the largest academic propulsion lab in the world.

The abstract reads, “Heterogeneous materials used in biomedical, structural and electronics applications contain a high fraction of solids (> 60 vo.%) and exhibit extremely high viscosities (μ > 1000 Pa·s), which hinders their 3D printing using existing technologies. This study shows that inducing high-amplitude ultrasonic vibrations within a nozzle imparts sufficient inertial forces to these materials to drastically reduce effective wall friction and flow stresses, enabling their 3D printing with moderate back pressures (< 1 MPa) at high rates and with precise flow control. This effect is utilized to demonstrate the printing of a commercial polymer clay, an aluminum-polymer composite and a stiffened fondant with viscosities up to 14,000 Pa·s with minimal residual porosity at rates comparable to thermoplastic extrusion. This new method can significantly extend the type of materials that can be printed to produce functional parts without relying on special shear/thermal thinning formulations or solvents to lower viscosity of the plasticizing component. The high yield strength of the printed material also allows free- form 3D fabrication with minimal need for supports.”

So far, the Purdue research team has been successful in 3D printing items using viscous materials with 100-micron precision – better than most consumer 3D printers currently available – and also managed to maintain high print rates.

“The most common form of 3D printing is thermoplastic extrusion. That’s usually good enough for prototypes, but for actual fabrication, you need to use materials with high strength, like ceramics or metal composites with a large fraction of solid particles,” Gundez explained. “The precursors for these materials are extremely viscous, and normal 3D printers can’t deposit them, because they can’t be pushed through a small nozzle.”

Visualizing this particular 3D printing process is not easy, because of the opaque materials and the fact that the surfaces are hidden inside the 3D printer’s nozzle.

“The results were really striking. Nobody has ever characterized a viscous flow through a channel this way,” Gundez said. “We were able to quantify the flow, and understand how our method was actually working.”

Graphical abstract

3D printing gives manufacturers the ability to customize the geometry of a rocket, as well as modify its combustion. So the first practical application the team is considering for its innovative method is solid rocket fuel, which has shown potential for 3D printing.

“Solid propellants start out very viscous, like the consistency of cookie dough. It’s very difficult to print because it cures over time, and it’s also very sensitive to temperature,” explained PhD candidate McClain. “But with this method, we were actually able to print strands of solid propellant that burned comparably to traditionally cast methods.”

McClain 3D printed some two-centimeter samples as a way to test the combustion. The samples were then ignited in a high-pressure (up to 1,000 pounds PSI) vessel, and the researchers analyzed a slow-motion video of the resulting burn.

“We may want to have certain parts burn faster or slower, or something that burns faster in the center than the outside. We can create this much more precisely with this 3D printing method,” McClain said.

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